Electrochemical conversion of anhydrous hydrogen halide to halogen gas using a cation-transporting membrane

ABSTRACT

The invention relates to a process for electrochemically converting anhydrous hydrogen halide, such as hydrogen chloride, hydrogen fluoride, hydrogen bromide and hydrogen iodide, to essentially dry halogen gas, such as chlorine, fluorine, bromine and iodine gas, respectively. In a preferred embodiment, the present invention relates to a process for electrochemically converting anhydrous hydrogen chloride to essentially dry chlorine gas. This process allows the production of high-purity chlorine gas. In this process, molecules of essentially anhydrous hydrogen chloride are transported through an inlet of an electrochemical cell. The molecules of the essentially anhydrous hydrogen chloride are oxidized at the anode of the cell to produce essentially dry chlorine gas and protons, which are transported through the membrane of the cell. The transported protons are reduced at the cathode to form either hydrogen gas or water.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for electrochemicallyconverting anhydrous hydrogen halide to an essentially dry halogen gasusing a cation-transporting membrane. In particular, this process may beused to produce halogen gas such as chlorine, bromine, fluorine andiodine from a respective anhydrous hydrogen halide, such as hydrogenchloride, hydrogen bromide, hydrogen fluoride and hydrogen iodide.

2. Description of the Related Art

Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product ofmany manufacturing processes which use chlorine. For example, chlorineis used to manufacture polyvinylchloride, isocyanates, and chlorinatedhydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as aby-product of these processes. Because supply so exceeds demand,hydrogen chloride or the acid produced often cannot be sold or used,even after careful purification. Shipment over long distances is noteconomically feasible. Discharge of the acid or chloride ions into wastewater streams is environmentally unsound. Recovery and feedback of thechlorine to the manufacturing process is the most desirable route forhandling the HCl by-product. A number of commercial processes have beendeveloped to convert HCl into usable chlorine gas. See e.g., F. R. Minz,“HCl-Electrolysis—Technology for Recycling Chlorine”, Bayer AG,Conference on Electrochemical Processing, Innovation & Progress,Glasgow, Scotland, UK, Apr. 21-Apr. 23, 1993.

Currently, thermal catalytic oxidation processes exist for convertinganhydrous HCl and aqueous HCl into chlorine. Commercial processes, knownas the “Shell-Chlor”, the “Kel-Chlor” and the “MT-Chlor” processes, arebased on the Deacon reaction. The original Deacon reaction as developedin the 1870's made use of a fluidized bed containing a copper chloridesalt which acted as the catalyst. The Deacon reaction is generallyexpressed as follows:

where the following catalysts may be used, depending on the reaction orprocess in which equation (1) is used.

Reaction Catalyst or Process Cu Deacon Cu, Rare Earth, AlkaliShell-Chlor NO₂, NOHSO₄ Kel-Chlor Cr_(m)O_(n) MT-Chlor

The commercial improvements to the Deacon reaction have used othercatalysts in addition to or in place of the copper used in the Deaconreaction, such as rare earth compounds, various forms of nitrogen oxide,and chromium oxide, in order to improve the rate of conversion, toreduce the energy input and to reduce the corrosive effects on theprocessing equipment produced by harsh chemical reaction conditions.However, in general these thermal catalytic oxidation processes arecomplicated because they require separating the different reactioncomponents in order to achieve product purity. They also involve theproduction of highly corrosive intermediates, which necessitatesexpensive construction materials for the reaction systems. Moreover,these thermal catalytic oxidation processes are operated at elevatedtemperatures of 250° C. and above.

Electrochemical processes exist for converting aqueous HCl to chlorinegas by passage of direct electrical current through the solution. Thecurrent electrochemical commercial process is known as the Uhde process.In the Uhde process, aqueous HCl solution of approximately 22% is fed at65° to 80° C. to both compartments of an electrochemical cell, whereexposure to a direct current in the cell results in an electrochemicalreaction and a decrease in HCl concentration to 17% with the productionof chlorine gas and hydrogen gas. A polymeric separator divides the twocompartments. The process requires recycling of dilute (17%) HClsolution produced during the electrolysis step and regenerating an HClsolution of 22% for feed to the electrochemical cell. The overallreaction of the Uhde process is expressed by the equation:

As is apparent from equation (2), the chlorine gas produced by the Uhdeprocess is wet, usually containing about 1% to 2% water. This wetchlorine gas must then be further processed to produce a dry, usablegas. If the concentration of HCl in the water becomes too low, it ispossible for oxygen to be generated from the water present in the Uhdeprocess. This possible side reaction of the Uhde process due to thepresence of water, is expressed by the equation:

2H₂O→O₂+4H⁺+4e⁻  (3)

Further, the presence of water in the Uhde system limits the currentdensities at which the cells can perform to less than 500 amps/ft.²,because of this side reaction The result is reduced electricalefficiency and corrosion of the cell components due to the oxygengenerated.

Another electrochemical process for processing aqueous HCl has beendescribed in U.S. Pat. No. 4,311,568 to Balko. Balko employs anelectrolytic cell having a solid polymer electrolyte membrane. Hydrogenchloride, in the form of hydrogen ions and chloride ions in aqueoussolution, is introduced into an electrolytic cell. The solid polymerelectrolyte membrane is bonded to the anode to permit transport from theanode surface into the membrane. In Balko, controlling and minimizingthe oxygen evolution side reaction is an important consideration.Evolution of oxygen decreases cell efficiency and leads to rapidcorrosion of components of the cell. The design and configuration of theanode pore size and electrode thickness employed by Balko maximizestransport of the chloride ions. This results in effective chlorineevolution while minimizing the evolution of oxygen, since oxygenevolution tends to increase under conditions of chloride ion depletionnear the anode surface. In Balko, although oxygen evolution may beminimized, it is not eliminated. As can be seen from FIGS. 3 to 5 ofBalko, as the overall current density is increased, the rate of oxygenevolution increases, as evidenced by the increase in the concentrationof oxygen found in the chlorine produced. Balko can run at highercurrent densities, but is limited by the deleterious effects of oxygenevolution. If the Balko cell were to be run at high current densities,the anode would be destroyed.

In general, the rate of an electrochemical process is characterized byits current density. In many instances, a number of electrochemicalreactions may occur simultaneously. When this is true, the electricaldriving force for electrochemical reactions is such that it results inan appreciable current density for more than one electrochemicalreaction. For these situations, the reported or measured current densityis a result of the current from more than one electrochemical reaction.This is the case for the electrochemical oxidation of aqueous hydrogenchloride. The oxidation of the chloride ions is the primary reaction.However, the water present in the aqueous hydrogen chloride is oxidizedto evolve oxygen as expressed in equation (3). This is not a desirablereaction. The current efficiency allows one to describe quantitativelythe relative contribution of the current from multiple sources. Forexample, if at the anode or cathode multiple reactions occur, then thecurrent efficiency can be expressed as: $\begin{matrix}{\eta_{j} = \frac{i_{j}}{\sum\limits_{j = 1}^{NR}t_{j}}} & (4)\end{matrix}$

where ηj is the current efficiency of reaction j, and where there are NRnumber of reactions occurring.

For the example of an aqueous solution of HCl and an anode, the generalexpression above is: $\begin{matrix}{\eta_{{Cl}_{2}} = \frac{i_{{Cl}_{2}}}{i_{{Cl}_{2}} + i_{O_{2}}}} & (5) \\{{\eta_{{Cl}_{2}} + \eta_{O_{2}}} = 1.0} & (6)\end{matrix}$

In the specific case of hydrogen chloride in an aqueous solution,oxidation of chloride is the primary reaction, and oxygen evolution isthe secondary reaction. In this case, the current density is the sum ofthe two anodic reactions. Since η_(O2) is not zero, the currentefficiency for chloride oxidation is less than unity, as expressed inequations (7) and (8) below. Whenever one is concerned with theoxidation of chloride from an aqueous solution, then the currentefficiency for oxygen evolution is not zero and has a deleterious effectupon the yield and production of chlorine.

η_(O2)≠0  (7)

η_(Cl2)=1.0−η_(O2) . . . i_(Cl2)=η_(Cl2)×i_(reported)  (8)

Furthermore, electrolytic processing of aqueous HCl can be mass-transferlimited. Mass-transfer of species is very much influenced by theconcentration of the species as well as the rate of diffusion. Thediffusion coefficient and the concentration of species to be transportedare important factors which affect the rate of mass transport. In anaqueous solution, such as that used in Balko, the diffusion coefficientof a species is ˜10⁻⁵ cm.²/sec. In a gas, the diffusion coefficient isdramatically higher, with values ˜10⁻² cm.²/sec. In normal industrialpractice for electrolyzing aqueous hydrogen chloride, the practicalconcentration of hydrogen chloride or chloride ion is ˜17% to 22%,whereas the concentration of hydrogen chloride is 100% in a gas ofanhydrous hydrogen chloride. Above 22%, conductance drops, and the powerpenalty begins to climb. Below 17%, oxygen can be evolved from water,per the side reaction of equation (3), corroding the cell components,reducing the electrical efficiency, and contaminating the chlorine.

SUMMARY OF THE INVENTION

The present invention solves the problems of the prior art by providinga process for the direct production of essentially dry halogen gas fromanhydrous hydrogen halide. This process allows for direct processing ofanhydrous hydrogen halide produced from many manufacturing processes,without first dissolving the hydrogen halide in water. This directproduction of essentially dry halogen gas, when done, for example, forchlorine gas, is less capital intensive than processes of the prior art,which require separation of water from the chlorine gas.

The present invention also solves the problems of the prior art byproviding a process in which chlorine is produced from a medium that isessentially water-free. Hence, in the electrochemical conversion ofhydrogen chloride (gas) to chlorine and hydrogen, no appreciable amountof oxygen is produced. Oxidation of water at the anode is an undesirableside reaction which is virtually eliminated in the present invention.Hence, the reaction can be run at higher current densities forconversion to chlorine, which translates into higher chlorine productionper unit area of electrode. Thus, the present invention requires lowerinvestment costs than the electrochemical conversions of hydrogenchloride of the prior art.

An advantage of using anhydrous hydrogen chloride in the presentinvention rather than aqueous hydrogen chloride is that the theoreticalcell voltage is lower by at least 0.3 V. This allows the cell to beoperated at lower overall cell voltages than cells operated with aqueoushydrogen chloride. This advantage can translate directly into lowerpower costs per pound of chlorine generated than in the aqueouselectrochemical processes of the prior art.

The present invention also provides a process which produces drierchlorine gas with fewer processing steps as compared to that produced byelectrochemical or catalytic systems of the prior art, therebysimplifying processing conditions and reducing capital costs.

The present invention also provides for a process for convertinganhydrous hydrogen chloride to essentially dry chlorine gas in order torecycle chlorine gas back to a manufacturing, or synthesis process,thereby eliminating environmental problems associated with the dischargeof chloride ions.

To achieve the foregoing solutions, and in accordance with the purposesof the invention as embodied and broadly described herein, there isprovided a process for the direct production of essentially dry halogengas from essentially anhydrous hydrogen halide, wherein molecules ofessentially anhydrous hydrogen halide are transported through an inletof an electrochemical cell comprising a cation-transporting membrane andan anode and a cathode each disposed in contact with a respective sideof the membrane; the molecules of the essentially anhydrous hydrogenhalide are oxidized at the anode to produce essentially dry halogen gasand protons; the protons are transported through the membrane of theelectrochemical cell; and the transported protons are reduced at thecathode.

In a preferred embodiment of the present invention, there is provided aprocess for the direct production of essentially dry chlorine gas fromessentially anhydrous hydrogen chloride, wherein molecules of anhydroushydrogen chloride are transported through an inlet of an electrochemicalcell comprising a cation-transporting membrane and an anode and acathode each disposed in contact with a respective side of the membrane;the molecules of the essentially anhydrous hydrogen chloride areoxidized at the anode to produce essentially dry chlorine gas andprotons; the protons are transported through the membrane of theelectrochemical cell; and the transported protons are reduced at thecathode. An oxygen-containing gas may be introduced at the cathode sideof the membrane, and when this is done, the protons and oxygen arereduced at the cathode side to form water.

It is likely that a portion of the anhydrous hydrogen chloride isunreacted after contacting the cell. In the present invention, theportion of the unreacted hydrogen chloride is separated from theessentially dry chlorine gas and is recycled back to the electrochemicalcell. In addition, the essentially dry chlorine gas may be fed to asynthesis process which produces anhydrous hydrogen chloride as aby-product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. is a schematic diagram of an electrochemical cell for producinghalogen gas from anhydrous hydrogen halide according to a firstembodiment of the present invention, which has a hydrogen-producingcathode.

FIG. 2 is a schematic view of an electrochemical cell for producinghalogen gas from anhydrous hydrogen halide according to a secondembodiment of the present invention, which has a water-producingcathode.

FIG. 3 is a schematic diagram of a system which separates a portion ofunreacted hydrogen chloride from the essentially dry chlorine gas andrecycles it back to the electrochemical cell of FIG. 1.

FIG. 4 is a schematic diagram of a modification to the system of FIG. 3which includes a synthesis process which produces anhydrous hydrogenchloride as a by-product and where the essentially dry chlorine gas isrecycled to the synthesis process, and the unreacted hydrogen chlorideis recycled back to the electrochemical cell of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention as illustrated in the accompanyingdrawings.

In accordance with the first embodiment of the present invention, thereis provided an electrochemical cell for the direct production ofessentially dry halogen gas from anhydrous hydrogen halide. This cellwill be described with respect to a preferred embodiment of the presentinvention, which directly produces essentially dry chlorine gas fromanhydrous hydrogen chloride. However, this cell may alternatively beused to produce other halogen gases, such as bromine, fluorine andiodine from a respective anhydrous hydrogen halide, such as hydrogenbromide, hydrogen fluoride and hydrogen iodide. The term “direct” meansthat the electrochemical cell obviates the need to remove water from thechlorine produced or the need to convert essentially anhydrous hydrogenchloride to aqueous hydrogen chloride before electrochemical treatment.Such a cell is shown generally at 10 in FIG. 1. In this firstembodiment, chlorine gas, as well as hydrogen, is produced by this cell.

Cell 10 comprises a cation-transporting membrane 12 as shown in FIG. 1.More specifically, membrane 12 may be a proton-conducting membrane.Membrane 12 can be a commercial cationic membrane made of a fluoro orperfluoropolymer, preferably a copolymer of two or more fluoro orperfluoromonomers, at least one of which has pendant sulfonic acidgroups. The presence of carboxylic groups is not desirable, becausethose groups tend to decrease the conductivity of the membrane when theyare protonated. Various suitable resin materials are availablecommercially or can be made according to patent literature. They includefluorinated polymers with side chains of the type —CF₂CFRSO₃H and—OCF₂CF₂CF₂SO₃H, where R is a F, Cl, CF₂Cl, or a C₁ to C₁₀perfluoroalkyl radical. The membrane resin may be, for example, acopolymer of tetrafluoroethylene with CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₃H.Sometimes those resins may be in the form that has pendant —SO₂F groups,rather than —SO₃H groups. The sulfonyl fluoride groups can be hydrolyzedwith potassium hydroxide to —SO₃K groups, which then are exchanged withan acid to —SO₃H groups. Suitable cationic membranes, which are made ofhydrated, copolymers of polytetrafluoroethylene and poly-sulfonylfluoride vinyl ether-containing pendant sulfonic acid groups, areoffered by E. I. du Pont de Nemours and Company of Wilmington, Del.under the trademark “NAFION” (hereinafter referred to as NAFION®). Inparticular, NAFION® membranes containing pendant sulfonic acid groupsinclude NAFION® 117, NAFION® 324 and NAFION® 417. The first type ofNAFION® is unsupported and has an equivalent weight of 1100 g.,equivalent weight being defined as the amount of resin required toneutralize one liter of a 1M sodium hydroxide solution. The other twotypes of NAFION® are both supported on a fluorocarbon fabric, theequivalent weight of NAFION® 417 also being 1100 g. NAFION® 324 has atwo-layer structure, a 125 μm-thick membrane having an equivalent weightof 1100 g., and a 25 μm-thick membrane having an equivalent weight of1500 g. There also is offered a NAFION® 117F grade, which is a precursormembrane having pendant SO₂F groups that can be converted to sulfonicacid groups.

Although the present invention describes the use of a solid polymerelectrolyte membrane, it is well within the scope of the invention touse other cation-transporting membranes which are not polymeric. Forexample, proton-conducting ceramics such as beta-alumina may be used.Beta-alumina is a class of nonstoichiometric crystalline compoundshaving the general structure Na₂O_(x).Al₂O₃, in which x ranges from 5(β″-alumina) to 11 (β-alumina). This material and a number of solidelectrolytes which are useful for the invention are described in theFuel Cell Handbook, A. J. Appleby and F. R. Foulkes, Van NostrandReinhold, N.Y., 1989, pages 308-312. Additional useful solid stateproton conductors, especially the cerates of strontium and barium, suchas strontium ytterbiate cerate (SrCe_(0.95)Yb_(0.05)O_(3-a)) and bariumneodymiate cerate (BaCe_(0.9)Nd_(0.01)O_(3-a)) are described in a finalreport, DOE/MC/24218-2957, Jewulski, Osif and Remick, prepared for theU.S. Department of Energy, Office of Fossil Energy, Morgantown EnergyTechnology Center by Institute of Gas Technology, Chicago, Ill.,December, 1990.

Electrochemical cell 10 also comprises a pair of electrodes,specifically, an anode 14 and a cathode 16, each disposed in contactwith a respective side of the membrane as shown in FIG. 1. Anode 14 hasan anode inlet 18 which leads to an anode chamber 20, which in turnleads to an anode outlet 22. Cathode 16 has a cathode inlet 24 whichleads to a cathode chamber 26, which in turn leads to a cathode outlet28. As known to one skilled in the art, if electrodes are placed onopposite faces of a membrane, cationic charges (protons in the HClreaction being described) are transported through the membrane fromanode to cathode, while each electrode carries out a half-cell reaction.In the present invention, molecules of anhydrous hydrogen chloride aretransported to the surface of the anode through inlet 18. The moleculesof the anhydrous hydrogen chloride are oxidized to produce essentiallydry chlorine gas and protons. The essentially dry chlorine gas exitsthrough anode outlet 22 as shown in FIG. 1. The protons, designated asH+in FIG. 1, are transported through the membrane and reduced at thecathode. This is explained in more detail below.

The anode and the cathode may comprise porous, gas-diffusion electrodes.Such electrodes provide the advantage of high specific surface area, asknown to one skilled in the art. The anode and the cathode comprise anelectrochemically active material disposed adjacent, meaning at orunder, the surface of the cation-transporting membrane. A thin film ofthe electrochemically active material may be applied directly to themembrane. Alternatively, the electrochemically active material may behot-pressed to the membrane, as shown in A. J. Appleby and E. B. Yeager,Energy, Vol. 11, 137 (1986). Alternatively, the electrochemically activematerial may be deposited into the membrane, as shown in U.S. Pat. No.4,959,132 to Fedkiw. The electrochemically active material may compriseany type of catalytic or metallic material or metallic oxide, as long asthe material can support charge transfer. Preferably, theelectrochemically active material may comprise a catalyst material suchas platinum, ruthenium, osmium, rhenium, rhodium, iridium, palladium,gold, titanium or zirconium and the oxides, alloys or mixtures thereof.The phrase “mixtures comprising any of these elements, oxides andalloys” means at least one of these elements, oxides and alloys mixedwith at least one of any other of these elements, oxides and alloysand/or any other constituent. However, in general, the oxides of thesematerials are not used for the cathode. Other catalyst materialssuitable for use with the present invention may include, but are notlimited to, transition metal macrocycles in monomeric and polymericforms and transition metal oxides, including perovskites and pyrochores.

In a hot-pressed electrode, the electrochemically active material maycomprise a catalyst material on a support material. The support materialmay comprise particles of carbon and particles ofpolytetrafluoroethylene, which is sold under the trademark “TEFLON”(hereinafter referred to as TEFLON®), commercially available from E. I.du Pont de Nemours and Company of Wilmington, Del. The electrochemicallyactive material may be bonded by virtue of the TEFLON® to a supportstructure of carbon paper or graphite cloth and hot-pressed to thecation-transporting membrane. The hydrophobic nature of TEFLON® does notallow a film of water to form at the anode. A water barrier in theelectrode would hamper the diffusion of HCl to the reaction sites. Theelectrodes are preferably hot-pressed into the membrane in order to havegood contact between the catalyst material and the membrane.

The loadings of electrochemically active material may vary based on themethod of application to the membrane. Hot-pressed, gas-diffusionelectrodes typically have loadings of 0.10 to 0.50 mg./cm.². Lowerloadings are possible with other available methods of deposition, suchas distributing them as thin films from inks onto the membranes, asdescribed in Wilson and Gottesfeld, “High Performance CatalyzedMembranes of Ultra-low Pt Loadings for Polymer Electrolyte Fuel Cells”Los Alamos National Laboratory, J. Electrochem. Soc., Vol. 139, No. 2L28-30, 1992, where the inks contain solubilized NAFION® ionomer toenhance the catalyst-ionomer surface contact and to act as a binder tothe NAFION® membrane sheet. With such a system, loadings as low as 0.017mg. active material per cm.² have been achieved.

A current collector 30, 32, respectively, is disposed in electricalcontact with the anode and the cathode, respectively, for collectingcharge. Another function of the current collectors is to directanhydrous hydrogen chloride to the anode and to direct any water addedto the cathode at inlet 24 to keep the membrane hydrated, as will bediscussed below. More specifically, the current collectors are machinedwith flow channels 34, 36 as shown in FIG. 1 for directing the anhydrousHCl to the anode and the water added to the cathode. It is within thescope of the present invention that the current collectors and the flowchannels may have a variety of configurations. Also, the currentcollectors may be made in any manner known to one skilled in the art.For example, the current collectors may be machined from graphite blocksimpregnated with epoxy to keep the hydrogen chloride and chlorine fromdiffusing through the block. This impregnation also prevents oxygen andwater from leaking through the blocks. The current collectors may alsobe made of a porous carbon in the form of a foam, cloth or matte. Thecurrent collectors may also include thermocouples or thermistors (notshown) to monitor and control the temperature of the cell.

The electrochemical cell of the first embodiment also comprises astructural support for holding the cell together. Preferably, thesupport comprises a pair of backing plates which are torqued to highpressures to reduce the contact resistances between the currentcollectors and the electrodes. The plates may be aluminum, but arepreferably a corrosion-resistant metal alloy. The plates include heatingelements (not shown) which are used to control the temperature of thecell. A non-conducting element, such as TEFLON® or other insulator, isdisposed between the collectors and the backing plates.

The electrochemical cell of the first embodiment also includes a voltagesource (not shown) for supplying a voltage to the cell. The voltagesource is attached to the cell through current collectors 30 and 32 asindicated by the + and − terminals, respectively, as shown in FIG. 1.

When more than one anode-cathode pair is used, such as in manufacturing,a bipolar arrangement is preferred. In the simple cell shown in FIG. 1,a single anode and cathode are shown. The current flows from theexternal voltage source to the cathode and returns to the externalsource through the lead connected to the anode. With the stacking ofnumerous anode-cathode pairs, it is not most convenient to supply thecurrent in this fashion. Hence, for a bipolar arrangement, the currentflows through the cell stack. This is accomplished by having the currentcollector for the anode and the cathode machined from one piece ofmaterial. Thus, on one face of the current collector, the gas (HCl) forthe anode flows in machined channels past the anode. On the other faceof the same current collector, channels are machined, and the current isused in the cathodic reaction, which produces hydrogen in thisinvention. The current flows through the repeating units of a cell stackwithout the necessity of removing and supplying current to eachindividual cell. The material selected for the current collector must beresistant to the oxidizing conditions on the anode side and the reducingconditions on the cathode side. 0f course, the material must beelectronically conductive. In a bipolar configuration, insulators arenot interspersed in the stack as described above. Rather, there arebacking plates at the ends of the stack, and these may be insulated fromthe adjacent current collectors.

Further in accordance with the first embodiment of the presentinvention, there is provided a process for the direct production ofessentially dry halogen gas from essentially anhydrous hydrogen halide.The anhydrous hydrogen halide may comprise hydrogen chloride, hydrogenbromide, hydrogen fluoride or hydrogen iodide. It should be noted thatthe production of bromine gas and iodine gas can be accomplished whenthe electrochemical cell is run at elevated temperatures (i.e., about60° C. and above for bromine and about 190° C. and above for iodine). Inthe case of iodine, a membrane made of a material other than NAFION®should be used.

The operation of the electrochemical cell of the first embodiment willnow be described as it relates to a preferred embodiment of the processof the present invention, where the anhydrous hydrogen halide ishydrogen chloride. In operation, molecules of essentially anhydroushydrogen chloride gas are transported to the surface of the anodethrough anode inlet 18 and through gas channels 34. Water (H₂O (1) asshown in FIG. 1) is delivered to the cathode through cathode inlet 24and through channels 36 formed in cathode current collector 32 tohydrate the membrane and thereby increase the efficiency of protontransport through the membrane. Molecules of the anhydrous hydrogenchloride (HCl(g) as shown in FIG. 1) are oxidized at the anode under thepotential created by the voltage source to produce essentially drychlorine gas (Cl₂(g)) at the anode, and protons (H+) as shown in FIG. 1.This reaction is given by the equation:

The chlorine gas (Cl₂(g)) exits through anode outlet 22 as shown in FIG.1. The protons (H⁺) are transported through the membrane, which acts asan electrolyte. The transported protons are reduced at the cathode. Thisreaction is given by the equation:

The hydrogen which is evolved at the interface between the electrode andthe membrane exits via cathode outlet 28 as shown in FIG. 1. Thehydrogen bubbles through the water and is not affected by the TEFLON® inthe electrode.

FIG. 2 illustrates a second embodiment of the present invention.Wherever possible, elements corresponding to the elements of theembodiment of FIG. 1 will be shown with the same reference numeral as inFIG. 1, but will be designated with a prime (′).

In accordance with the second embodiment of the present invention, thereis provided an electrochemical cell for the direct production ofessentially dry halogen gas from anhydrous hydrogen halide. This cellwill be described with respect to a preferred embodiment of the presentinvention, which directly produces essentially dry chlorine gas fromanhydrous hydrogen chloride. However, this cell may alternatively beused to produce other halogen gases, such as bromine, fluorine andiodine from a respective anhydrous hydrogen halide, such as hydrogenbromide, hydrogen fluoride and hydrogen iodide. Such a cell is showngenerally at 10′ in FIG. 2. In this second embodiment, water, as well aschlorine gas, is produced by this cell.

Cell 10′ comprises a cation-transporting membrane 12′ as shown in FIG.2. Membrane 12′ may be a proton-conducting membrane. Preferably,membrane 12′ comprises a solid polymer membrane, and more preferably thepolymer comprises NAFION® as described above with respect to the firstembodiment.

Electrochemical cell 10′ also comprises a pair of electrodes,specifically, a anode 14′ and a cathode 16′, each disposed in contactwith a respective side of the membrane as shown in FIG. 2. Anode 14′ andcathode 16′ function as described above with respect to the firstembodiment. Anode 14′ has an inlet 18′ which leads to an anode chamber20′, which in turn leads to an outlet 22′. Cathode 16′ has an inlet 24′which leads to a cathode chamber 26′, which in turn leads to an outlet28′.

As in the first embodiment, the anode and the cathode may compriseporous electrodes, and more specifically, gas-diffusion electrodes whichmay be constructed and comprise materials as described above in thefirst embodiment.

The electrochemical cell of the second embodiment of the presentinvention also comprises a current collector 30′, 32′ disposed inelectrical contact with the anode and the cathode, respectively, forcollecting charge. The current collectors are machined with flowchannels 34′, 36′ as shown in FIG. 2 for directing the anhydrous HCl tothe anode and the oxygen (O₂) to the cathode. The current collectors areconstructed and function as described above with respect to the firstembodiment. In addition to collecting charge, another function of thecurrent collectors in this second embodiment is to direct anhydroushydrogen chloride across the anode. The cathode current collectordirects the oxygen-containing gas, which may contain water vapor as theresult of humidification, to the cathode. Water vapor may be needed tokeep the membrane hydrated. However, water vapor may not be necessary inthis embodiment because of the water produced by the electrochemicalreaction of the oxygen (O₂) added as discussed below.

The electrochemical cell of the second embodiment also comprises astructural support for holding the cell together. Preferably, thesupport comprises a pair of backing plates (not shown) which areconstructed and which function as described above with respect to thefirst embodiment.

The electrochemical cell of the second embodiment also includes avoltage source (not shown) for supplying a voltage to the cell. Thevoltage source is attached to the cell through current collectors 30′and 32′ as indicated by the + and − terminals, respectively, as shown inFIG. 2.

Further in accordance with the second embodiment of the presentinvention, there is provided a process for the direct production ofessentially dry halogen gas from essentially anhydrous hydrogen halide.The anhydrous hydrogen halide may comprise hydrogen chloride, hydrogenbromide, hydrogen fluoride or hydrogen iodide. It should be noted thatthe production of bromine gas and iodine gas can be accomplished whenthe electrochemical cell is run at elevated temperatures (i.e., about60° C. and above for bromine and about 190° C. and above for iodine). Inthe case of iodine, a membrane made of a material other than NAFION®should be used.

The operation of the electrochemical cell of the second embodiment willnow be described as it relates to a preferred embodiment of the processof the present invention, where the anhydrous hydrogen halide ishydrogen chloride. In operation, molecules of essentially anhydroushydrogen chloride are transported to the anode through anode inlet 18′and through gas channels 34′. An oxygen-containing gas, such as oxygen(O₂(g) as shown in FIG. 2), air or oxygen-enriched air (i.e., greaterthan 21 mol % oxygen in nitrogen) is introduced through cathode inlet24′ as shown in FIG. 2 and through channels 36′ formed in the cathodecurrent collector. Although air is cheaper to use, cell performance isenhanced when enriched air or oxygen is used. This cathode feed gas maybe humidified to aid in the control of moisture in the membrane.Molecules of the hydrogen chloride (HCl(g)) as shown in FIG. 2) areoxidized under the potential created by the voltage source to produceessentially dry chlorine gas at the anode, and protons (H+) as shown inFIG. 2, as expressed in equation (9) above. The chlorine gas (Cl₂) exitsthrough anode outlet 22′ as shown in FIG. 2. The protons (H⁺) aretransported through the membrane, which acts as an electrolyte. Oxygenand the transported protons are reduced at the cathode to water, whichis expressed by the equation:

½O₂(g)+2e⁻+2H⁺→H₂O (g)  (11)

The water formed (H₂O (g) in equation (11)) exits via cathode outlet 28′as shown in FIG. 2, along with any nitrogen and unreacted oxygen. Thewater also helps to maintain hydration of the membrane, as will befurther explained below.

In this second embodiment, the cathode reaction is the formation ofwater. This cathode reaction has the advantage of more favorablethermodynamics relative to H₂production at the cathode as in the firstembodiment. This is because the overall reaction in this embodiment,which is expressed by the following equation:

involves a smaller free-energy change than the free-energy change forthe overall reaction in the first embodiment, which is expressed by thefollowing equation:

Thus, the amount of voltage or energy required as input to the cell isreduced in this second embodiment.

The membrane of both the first and the second embodiments must behydrated in order to have efficient proton transport. Thus, the processof either embodiment of the present invention includes the step ofkeeping the cathode side of the membrane moist to increase theefficiency of proton transport through the membrane. In the firstembodiment, which has a hydrogen-producing cathode, the hydration of themembrane is obtained by keeping liquid water in contact with thecathode. The liquid water passes through the gas-diffusion electrode andcontacts the membrane. In the second embodiment, which has awater-producing cathode, the membrane hydration is accomplished by theproduction of water as expressed by equation (11) above and by the waterintroduced in a humidified oxygen-feed or air-feed stream. This keepsthe conductivity of the membrane high.

In either of the first or second embodiments, the electrochemical cellcan be operated over a wide range of temperatures. Room temperatureoperation is an advantage, due to the ease of use of the cell. However,operation at elevated temperatures provides the advantages of improvedkinetics and increased electrolyte conductivity. It should be noted alsothat one is not restricted to operate the electrochemical cell of eitherthe first or the second embodiment at atmospheric pressure. The cellcould be run at differential pressure gradients, which change thetransport characteristics of water or other components in the cell,including the membrane.

The electrochemical cell of either embodiment of the present inventioncan be operated at higher temperatures at a given pressure thanelectrochemical cells operated with aqueous hydrogen chloride of theprior art. This affects the kinetics of the reactions and theconductivity of the NAFION®. Higher temperatures result in lower cellvoltages. However, limits on temperature occur because of the propertiesof the materials used for elements of the cell. For example, theproperties of a NAFION® membrane change when the cell is operated above120° C. The properties of a polymer electrolyte membrane make itdifficult to operate a cell at temperatures above 150° C. With amembrane made of other materials, such as a ceramic material likebeta-alumina, it is possible to operate a cell at temperatures above200° C.

In either the first or the second embodiment of the present invention, aportion of the anhydrous hydrogen chloride may be unreacted aftercontacting the cell and may exit the cell through the anode outlet alongwith the chlorine gas. This concept is illustrated with respect to FIG.3, where a system for recycling unreacted anhydrous hydrogen chloridefrom essentially dry chlorine gas is shown generally at 40. It should benoted that the system of FIG. 3 could be used to recycle other unreactedanhydrous hydrogen halides from a respective essentially dry halogengas, such as fluorine, bromine or iodine, chlorine gas being used onlyas a representative halogen gas. The system of FIG. 3 recycles theunreacted anhydrous hydrogen chloride back to cell 10 of the firstembodiment, which includes membrane 12, anode 14, anode chamber 20,cathode 16 and cathode chamber 26 as described above. Cell 10 alsoincludes current collectors 30, 32 having flow channels 34, 36 formedtherein. Cell 10 also includes a feed line 38 for feeding anhydroushydrogen chloride and a feed line 39 for feeding water, as describedabove for the first embodiment. The unreacted portion of the anhydrousHCl is separated from the essentially dry chlorine gas by a separator 44in a separation process which may involve distillation, adsorption,extraction, membrane separation or any number of known separationtechniques. The separated, unreacted portion of anhydrous HCl in theessentially dry chlorine gas is recycled through a line 45 as shown inFIG. 3 back to anode inlet 18 of electrochemical cell 10 as shown inFIG. 3. The separated chlorine gas exits through a line 46. In thesystem of FIG. 3, hydrogen gas (H₂) exits cell 10 through cathode outlet28 as described with respect to the first embodiment and through a line48. Excess water may also exit through cathode outlet 28, where it isseparated from hydrogen gas in a knock-out tank 49 and recycled tocathode inlet 24 through a line 41. The separated hydrogen gas exitsthrough a line 47. It should be understood that the cell of the secondembodiment of the present invention alternatively could be used in thesystem of FIG. 3, except that oxygen gas (O₂) would enter the cathodeinlet from feed line 39, and water in the form of gas (H₂O(g)), alongwith any nitrogen and unreacted oxygen, would exit the cathode outlet.

A modification of the system as shown in FIG. 3 above involves recyclingthe essentially dry chlorine gas which has been separated from theunreacted anhydrous hydrogen chloride to a synthesis process wherechlorine is a reactant and anhydrous hydrogen chloride is a by-product.This modification is illustrated in FIG. 4, where a system whichrecycles separated chlorine gas to a synthesis process is showngenerally at 50. System 50 includes system 40 as described above, aswell as a synthesis process 52 and other components associated therewithas described below. Essentially dry chlorine gas is recycled through aline 46 as described above to synthesis process 52. Other reactant inletlines are shown at 54 and 56. For instance, in a hydrofluorinationprocess, inlet line 54 could bring in hydrocarbon, and inlet line 56could bring in hydrogen fluoride (HF). Fluorinated hydrocarbons,unreacted HF and anhydrous hydrogen chloride exit process 52 through aline 58 and are separated in a separator 60 by any known separationprocess. The anhydrous hydrogen chloride is fed to anode inlet 18through a line 68 and is combined with a recycled stream in line 45 asshown in FIG. 4. Fluorinated hydrocarbons and unreacted HF exitseparator 60 via a line 62 and flow to a further separator 64, whichseparates the fluorinated hydrocarbons from the unreacted HF. Thefluorinated hydrocarbons exit separator 64 through a line 66. Theunreacted HF is recycled back to synthesis process 52 through a line 67,which joins up with inlet line 56. This system could also be used forbringing in hydrochlorofluorocarbons or chlorofluorocarbons plushydrogen and a hydrodechlorination catalyst to produce hydrogenchloride. It is, of course, within the scope of the present inventionalternatively to use the cell of the second embodiment in the system ofFIG. 4 with the differences to the system as noted above.

The invention will be clarified by the following Examples, which areintended to be purely exemplary of the invention. In the Examples givenbelow, experimental data are presented which show some of the resultsthat have been obtained by operating the first embodiment of the presentinvention for different electrode materials, temperatures, and differentmodes of operation. More specifically, in these experiments, the currentand the cell potential were measured for three different temperaturesand for two different electrode materials.

The electrode/membrane assemblies used in the following Examples arecommercially available from Giner, Inc. of Waltham, Massachusetts, asmembrane and electrode assemblies (MEAs) containing 0.35 mg. preciousmetal per cm.² and integrally bonded to NAFION® 117 membrane in theH+form. Electrodes as described in U.S. Pat. No. 4,210,501 may also beused with the present invention. It is also within the scope of thepresent invention to use other known metallization techniques in makingelectrodes for use with the present invention. For example, thetechnique used in U.S. Pat. No. 4,959,132 to Fedkiw, frequently referredto as Impregnation-Reduction, is an appropriate method to use with thepresent invention. Alternatively, the metallization technique may beused, as described in Japanese Publication No. 38934, published in 1980and J. Hydrogen Energy, 5, 397 (1982).

EXAMPLE 1

In this Example, a non-steady state electrochemical experiment (i.e., ofa duration of five minutes for each potential setting) generatingchlorine and hydrogen was performed in an electrochemical cell which was1 cm.×1 cm. in size. Platinum (Pt) extended with carbon was used for thecathode, and ruthenium oxide (RuO₂) extended with carbon was utilized inthe anode. The anode and cathode each contained 0.35 mg./cm.² preciousmetal. The anode and the cathode were both bonded to the membrane, whichwas made of NAFION® 117. The potential from the power source was steppedin 0.10 volt increments from 1.0 to 2.0 volts. At each 0.10 voltincrement, the potential was maintained for five minutes. The currentresponse at the specific cell potentials was recorded at three differenttemperatures, namely 40° C., 60° C. and 80° C., in order to assess theimportance of this variable upon cell performance and is given in Table1 below.

TABLE 1 Current Value Cell Potential [mAmp./cm²] [volts] 40° C. 60° C.80° C. 1.1 24 35 10 1.2 112 140 55 1.3 250 265 135 1.4 380 410 240 1.5540 540 340 1.6 585 620 440 1.7 640 710 540 1.8 660 760 630 1.9 700 7002.0 770 750

EXAMPLE 2

In this Example, a steady state electrochemical experiment (i.e., of aduration of 7 to 20 hours for each potential setting) generatingchlorine and hydrogen was performed in an electrochemical cell which was1 cm.×1 cm. in size. As in Example 1, platinum (Pt) extended with carbonwas used for the cathode, and ruthenium oxide (RuO₂) extended withcarbon was utilized in the anode. Also as in Example 1, the anode andcathode each contained 0.35 mg./cm.² precious metal, and the anode andthe cathode were both bonded to the NAFION® 117 membrane. The cellpotential from the power source was held at specific values for a longtime period, i.e., 7 to 20 hours. Again, the current response at thespecific cell potentials was recorded at three different temperatures,namely 40° C., 60° C. and 80° C. and is given in Table 2 below.

TABLE 2 Current Value Cell Potential [mAmp./cm²] [volts] 40° C. 60° C.80° C. 1.4 200 210 210 1.5 310 250 250 1.6 380 380 1.7 470 460 1.8 540

EXAMPLE 3

In this Example, a non-steady state electrochemical experiment (i.e., ofa duration of five minutes for each potential setting) generatingchlorine and hydrogen was performed in an electrochemical cell which was1 cm.×1 cm. in size, as in the above Examples. In this Example, platinum(Pt) extended with carbon was used for both the anode and the cathode.The anode and cathode each contained 0.35 mg./cm.² precious metal. Theanode and the cathode were both bonded to the membrane, which was madeof NAFION® 117. The potential from the power source was stepped in 0.10volt increments from 1.0 to 2.0 volts. At each 0.10 volt increment, thepotential was maintained for five minutes. The current response at thespecific cell potentials was recorded at two different temperatures,namely 40°0C. and 60° C., and is given in Table 3 below.

TABLE 3 Current Value Cell Potential [mAmp./cm²] [volts] 40° C. 60° C.1.1 25 30 1.2 96 125 1.3 200 260 1.4 310 395 1.5 425 500 1.6 520 610 1.7600 685 1.8 670 720 1.9 725 760 2.0 780 780

EXAMPLE 4

In this Example, a steady state electrochemical experiment (i.e., of aduration of 7 to 20 for each potential setting) generating chlorine andhydrogen was performed in an electrochemical cell which was 1 cm.×1 cm.in size, as in the above Examples. In this Example, platinum (Pt)extended with carbon was used for both the anode and the cathode. Theanode and cathode each contained 0.35 mg./cm.² precious metal. The anodeand the cathode were both bonded to the membrane, which was made ofNAFION® 117. In this Example, the cell potential from the power sourcewas held at specific values for a long time period, i.e., 7 to 20.Again, the current response at the specific cell potentials was recordedat three different temperatures, namely 40° C., 60° C. and 80° C. and isgiven in Table 4 below.

TABLE 4 Current Value Cell Potential [mAmp./cm²] [volts] 40° C. 60° C.80° C. 1.3 240 1.4 300 350 330 1.5 400 450 380 1.6 495 520 1.7 560

The results of these Examples indicate electrochemical cell performancewhich can exceed that generally obtained in the prior art.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention, in its broader aspects, is,therefore, not limited to the specific details, representative apparatusand illustrative Examples shown and described. Accordingly, departuresmay be made from such details without departing from the spirit or scopeof the general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A process for the direct production ofessentially dry halogen gas from essentially anhydrous hydrogen halide,wherein: (a) molecules of essentially anhydrous hydrogen halide are fedto an inlet of an electrochemical cell and are transported to an anodeof the cell; (b) the molecules of the essentially anhydrous hydrogenhalide are oxidized at the anode to produce essentially dry halogen gasand protons; (c) the protons are transported through acation-transporting membrane of the electrochemical cell; and (d) thetransported protons are reduced at a cathode of the electrochemicalcell.
 2. The process of claim 1, wherein the hydrogen halide-producinghalogen gas is selected from the group consisting of: hydrogen chloride,hydrogen bromide, hydrogen fluoride and hydrogen iodide.
 3. A processfor the direct production of essentially dry chlorine gas fromessentially anhydrous hydrogen chloride, wherein: (a) molecules ofessentially anhydrous hydrogen chloride are fed to an inlet of anelectrochemical cell and are transported to an anode of the cell; (b)the molecules of the essentially anhydrous hydrogen chloride areoxidized at the anode to produce essentially dry chlorine gas andprotons; (c) the protons are transported through a cation-transportingmembrane of the electrochemical cell; and (d) the transported protonsare reduced at a cathode of the electrochemical cell.
 4. The process ofclaims 1 or 3, wherein the transported protons are reduced to formhydrogen gas.
 5. The process of claim 1 or 3, wherein a gas containingoxygen is introduced at the cathode side of the membrane and the protonsand oxygen are reduced at the cathode side to form water.
 6. The processof claim 5, wherein the oxygen-containing gas comprises one of: air,oxygen, and oxygen-enriched air.
 7. A process for recycling unreactedanhydrous hydrogen halide generated from the direct production ofessentially dry halogen gas from essentially anhydrous hydrogen halide,wherein: (a) molecules of essentially anhydrous hydrogen halide are fedto an inlet of an electrochemical cell and are transported to an anodeof the cell; (b) a portion of the essentially anhydrous hydrogen halideis oxidized at the anode to produce essentially dry halogen gas andprotons; (c) the protons are transported through a cation-transportingmembrane of the electrochemical cell; (d) the transported protons arereduced at a cathode of the electrochemical cell; (e) another portion ofthe essentially anhydrous hydrogen halide is unreacted and is separatedfrom the essentially dry halogen gas; and (f) the unreacted, separatedportion of the anhydrous hydrogen halide exits the cell through anoutlet thereof and is recycled to the inlet of the electrochemical cell.8. The process of claim 7, wherein the essentially dry halogen gas isrecycled to a synthesis process which produces anhydrous hydrogen halideas a by-product.
 9. A process for recycling essentially dry halogen gasto a synthesis process, where the essentially dry halogen gas isgenerated directly from essentially anhydrous hydrogen halide, wherein:(a) molecules of essentially anhydrous hydrogen halide are fed to aninlet of an electrochemical cell and are transported to an anode of thecell; (b) a portion of the essentially anhydrous hydrogen halide isoxidized at the anode to produce essentially dry halogen gas andprotons; (c) the protons are transported through a cation-transportingmembrane of the electrochemical cell; (d) the transported protons arereduced at a cathode of the electrochemical cell; and (e) theessentially dry halogen gas is recycled to a synthesis process whichproduces anhydrous hydrogen halide as a by-product.
 10. Anelectrochemical cell for the direct production of essentially dryhalogen gas from essentially anhydrous hydrogen halide, comprising: (a)means for oxidizing molecules of essentially anhydrous hydrogen halideto produce essentially dry halogen gas and protons; (b)cation-transporting means for transporting the protons therethrough,wherein the oxidizing means is disposed in contact with one side of thecation-transporting means; and (c) means for reducing the transportedprotons, wherein the reducing means is disposed in contact with theother side of the cation-transporting means.
 11. The electrochemicalcell of claim 10, wherein the oxidizing means is an anode and thereducing means is a cathode and the cation-transporting means is acation-transporting membrane, and further wherein the anode and thecathode comprise gas-diffusion electrodes.
 12. The electrochemical cellof claim 10, wherein the oxidizing means is an anode, the reducing meansis a cathode, and the cation-transporting means is a cation-transportingmembrane, and further wherein the anode and the cathode comprise anelectrochemically active material disposed adjacent to a surface of thecation-transporting membrane.
 13. The electrochemical cell of claim 12,wherein a thin film of the electrochemically active material is applieddirectly to the membrane on opposite surfaces thereof.
 14. Theelectrical cell of claim 12, wherein the electrochemically activematerial is deposited into the membrane on opposite surfaces thereof.15. The electrical cell of claim 12, wherein the electrochemicallyactive material of the anode and the cathode comprises a catalystmaterial on a support material.
 16. The electrical cell of claim 15,wherein the support material comprises carbon.
 17. The electrochemicalcell of claim 15, wherein the catalyst material comprises one of thefollowing: platinum, ruthenium, osmium, rhenium, rhodium, iridium,palladium, gold, titanium and zirconium, and the oxides, alloys andmixtures thereof.
 18. A process for the direct production of essentiallydry halogen gas from essentially anhydrous hydrogen halide, wherein: (a)molecules of essentially anhydrous hydrogen halide are fed to an inletof an electrochemical cell and are transported to an anode of the cell;(b) the molecules of the essentially anhydrous hydrogen halide areoxidized at the anode to produce essentially dry halogen gas andprotons; (c) the protons are transported through a cation-transportingmembrane of the electrochemical cell to a cathode, the membrane havingan anode side and a cathode side; (d) a gas containing oxygen isintroduced at the cathode side of the membrane; and (e) the transportedprotons are reduced at the cathode, and the protons and oxygen arereduced at the cathode to form water.
 19. The process of claims 1,3, or18, further including the step of keeping the cathode side of themembrane moist to increase the efficiency of proton transport throughthe membrane.
 20. The process of claims 1, 3 or 18 wherein the cathodeand the anode comprise gas-diffusion electrodes.
 21. The process ofclaim 1,3 or 19, wherein the anode and the cathode comprise anelectrochemically active material disposed adjacent to the surface ofthe cation-transporting membrane.
 22. The process of claim 21, wherein athin film of the electrochemically active material is applied directlyto the membrane.
 23. The process of claim 21, wherein theelectrochemically active material is deposited into the membrane. 24.The process of claim 21, wherein the electrochemically active materialcomprises a catalyst material on a support material.
 25. The process ofclaim 24, wherein the support material comprises carbon.
 26. The processof claim 15, wherein the catalyst material comprises one of thefollowing: platinum, ruthenium, osmium, rhenium, rhodium, iridium,palladium, gold, titanium and zirconium, and the oxides, alloys andmixtures thereof.
 27. The process of claim 18, wherein theoxygen-containing gas comprises one of the following: air, oxygen andoxygen-enriched air.